1. Field of the Invention
This invention relates to a circuit arrangement for regulating the amplitude of a sawtooth wave generator, and more particularly to an arrangement in which a capacitor is periodically charged and discharged by a current source via a switch, the switch being controlled by a synchronizing signal.
2. Description of the Prior Art
A sawtooth generator of the type mentioned above produces a sawtooth signal in a very simple manner. The capacitor is linearly charged by the current source. At the end of a sawtooth period the capacitor is rapidly discharged, whereupon the charging process is reinitiated. This process is controlled by the switch, i.e. by the pulses of the synchronizing signal which controls the switch. If the frequency of the switch, and thus that of the periodic discharge changes, the charging of the capacitor is terminated at different values of amplitude. The amplitude of the sawtooth voltage at the output of the sawtooth generator is thus dependent upon frequency. If, for example, the frequency of the switch increases, the capacitor has less time to become charged. The charge is terminated at a lower value and thus the amplitude of the sawtooth voltage becomes lower. This fact is represented in FIG. 1 of the drawing in which the interval a between two pulses of a synchronizing signal S produces a greater amplitude of the sawtooth voltage V than the synchronizing pulses with the shorter interval b therebetween, and thus with a higher frequency.
In many applications this can result in undesirable consequences. For example, in the case of the production of the sawtooth signal for the operation of the vertical deflecting stage in television receivers, this means that a modified picture frequency produces a change in the picture height. If the vertical oscillator is in the form of a sawtooth generator whose sawtooth signal rises with a constant gradient between two fixed trigger limits, then in an unsynchronized state the sawtooth signal possesses a no-load frequency which, in practical situations, lies below the synchronizing frequency. European manufacturers fix the no-load frequency at approximately 44 Hz, the synchronizing frequency amounting to 50 Hz in Europe, and 60 Hz in the United States and in Japan. If the vertical oscillator is synchronized by the vertical synchronizing pulses, the last component of the sawtooth signal is cut off before the upper trigger threshold is reached, as is represented in FIG. 1 in respect of the interval b between the synchronizing pulses. As a consequence, not only is the amplitude of the sawtooth signal reduced, but in addition the dc voltage mean value is displaced. The reduction in the sawtooth signal amplitude in the case of synchronized operation in comparison to unsynchronized operation, such as can occur on the transfer of channels or, e.g. in the case of the breakdown of a transmitter even for a long period of time, indicates that the vertical output stage must be designed in accordance with the current which in this unfavorable situation is of a greater magnitude. The shift in the mean value produces an unstationary state which becomes manifest in a build-up of the picture state. The picture thereby exhibits a disturbing luffing in the vertical direction.
Multi-standard equipment demand different vertical frequencies, using a vertical oscillator which oscillates at the lowest occurring frequency and which is synchronized with the aid of the received vertical synchronizing pulses to a higher frequency, depending upon the standard. This would mean that whenever, for example, vertical frequencies are to be processed, which can differ by up to 20%, the picture height would likewise differ by 20%.